1. Field of the Invention
The invention relates to an illumination system for supplying an optical radiation beam along a principal axis and intended for illuminating an object which, in a plane perpendicular to the principal axis, has a non-round cross-section, said system comprising a radiation source, a concave reflector for concentrating radiation emitted by the radiation source and a lens system arranged in the path of the concentrated radiation. The invention also relates to an image projection apparatus comprising such an illumination system for illuminating at least one image display panel by means of which an image is generated which is projected on a projection screen via a projection lens system.
2. Description of the Related Art
The term image projection apparatus is herein used in its widest sense and comprises a device for displaying, for example, a video image, a graphic image, numerical information or a combination thereof. The image may be both a monochrome image and a color image. In the latter case the display system may comprise three color channels for, for example, the primary colors red, green and blue, each channel comprising a display panel. A monochrome image is understood to mean an image having only one color which in principle corresponds to a specific wavelength and in practice to a wavelength band around this specific, central wavelength. Such an image may be formed by a so-called monochromatic beam having a central wavelength and possibly a wavelength band surrounding it. The projection screen may be a transmission screen which may form part of the apparatus and close this apparatus at one side. It is alternatively possible for the screen to be arranged at some distance from the apparatus and it may be a reflecting screen in the form of, for example, a wall having a suitable reflection.
European Patent Application No. 0,293,007 describes a projection television apparatus in which an illumination system is used for illuminating a liquid crystal display panel. In the known illumination system the reflector is a parabolic reflector which encloses the radiation source through an angle of more than 180.degree. so that the radiation which is not directed towards the display panel is largely captured and as yet reflected to the display panel. When used in transmission a liquid crystal display panel has a low efficiency: for example, only 10% of the radiation incident on the panel is passed to the projection lens system. Moreover, this system projects the panel in a magnified form so that the illumination beam must have a large intensity if the radiation intensity per surface area unit on the screen is still to be sufficient. Since a projection television apparatus is a consumer apparatus which should be compact and inexpensive and have a construction which is as simple as possible, inter alia a simple cooling of the radiation source should suffice thus the radiation energy of the source should be limited and this energy should be used as efficiently as possible.
An important aspect is that the combination of radiation source and parabolic reflector supplies a parallel beam with a round cross-section, whereas the display panel is rectangular having a width (b)-height (h) ratio (aspect ratio) of, for example 4:3. For a complete illumination of the display panel the cross-section of the round beam at the location of the panel should have a diameter ##EQU1## and the center of the beam should coincide with the center of the display panel. The portions of the illumination beam outside the rectangle of the display panel are blocked and cannot be used for projecting the panel on the projection screen. To make better use of the available radiation, a transparent plate whose side facing the radiation source has a so-called linear Fresnel lens structure is arranged between the radiation source and the display panel in a first embodiment of a device according to European Patent Application No. 0,239,007. This structure comprises a plurality of cylindrical lenses whose longitudinal direction is parallel to the width direction of the display panel. The cylindrical lenses converge the beam in the direction of height of the panel so that in this direction more radiation passes through the panel. A second similar plate may be arranged in the radiation path behind the first Fresnel plate so as to cause the beam to be incident on the display panel as a parallel beam. In this embodiment the beam cross-section in the width direction of the display panel is not adapted, which results in a given quantity of radiation being lost in this direction. Moreover, since the beam supplied by the radiation source has a larger light intensity in the center than at the edge, the illumination intensity distribution on the display panel will not be uniform.
In a second embodiment of the illumination system according to European Patent Application 0,239,007 a more uniform distribution of radiation is realized because two circular Fresnel lens structures are used instead of the two linear Fresnel lens structures covering the complete plate surface areas. These circular Fresnel lens structures only cover an annular region of the plates. The first Fresnel lens structure converges the peripheral portion of the beam and the second Fresnel lens structure ensures that this peripheral portion will be parallel to the central portion of the beam which is not influenced by the plates.